A number of known methods and measuring instruments exist for determining the biometric data of an eye. For example, it is necessary to determine various biometric parameters of the eye prior to an operation to replace the lens of the eye if there is a clouding of the lens (cataract). To guarantee the optimal post-procedural visual acuity, these parameters must be determined with great accuracy. The appropriate replacement lens is selected based upon established formulae and calculation methods.
The most important parameters to be determined are, among others, the axial length (distance to the retina), the curvature and power of refraction of the cornea as well as the length of the anterior chamber (distance to the eye lens). These measurements can be determined successively using various opthalmologic devices or with the help of specially optimized biometric measuring systems.
For the determination of these parameters primarily ultrasound measuring devices and optical measuring devices based upon short coherence-light procedures prevailed.
With the ultrasound devices there are two different designs that function either based upon the “A-scan” principle or upon the “B-scan” principle. While the A-scan provides only one measurement in the axial direction, there is an additional measurement in transverse direction with the B-scan. The ultrasound procedure basically requires direct contact with the eye.
In this context a device for examining the eye, especially the human eye, is described in DE 42 35079 C2 that basically has the shape of a truncated cone in a shape matched to the eye which contains a probe for the evaluation of acoustic (ultrasound) signals. The probe is affixed at an oblique angle to the central axis of the holder and is suitable for transmitting as well as for receiving pulsed signals.
The specific disadvantages of the determination of the biometric data of an eye using ultrasound devices are, on one hand, the lesser accuracy and, on the other hand, the requirement of direct contact with the eye. This way the measurements could be distorted through denting of the eyeball. These disadvantages can be reduced through the use of the immersion technique where ultrasound waves are directed at the eye through a funnel filled with water and placed over the eye, but the major disadvantages of this measuring method remain.
These lie, on one hand, in the necessity of direct contact with the eye which always carries the risk of transmission of infections and, on the other hand, it is necessary to anesthetize the eye for the determination of the data. For the correct selection of the replacement lens it must be ascertained that the visual axis of the eye is appropriately aligned when determining the biometric data. For this purpose special devices must be provided for the ultrasound equipment since the alignment of the visual axis does not happen automatically.
Analogous to the ultrasound devices, where images of the structural transitions can be reconstructed based upon the acoustic signals, optical images of the structural transitions are depicted as two-dimensional depth tomograms. In this regard the OCT procedure (OCT=optical coherence tomography) has prevailed as a short coherence-light procedure where temporal incoherent light is used with the help of an interferometer for measuring the distance of reflective and dispersive materials.
The underlying principle of the OCT procedure is based upon white light interferometry and compares the travel time of a signal using an interferometer (in most cases a Michelson interferometer). The arm with a known optical length (=reference arm) is used as a reference arm for the measuring arm. The interference of the signals from both arms yields a pattern from which one can determine the relative optical travel distance within an A-scan (individual depth signal). In the one-dimensional scanning grid procedure the beam is guided transversally in one or two directions, analogous to the ultrasound technique, allowing the recording of a plane B-scan or a three-dimensional tomogram (C-scan). This way, the amplitude data of the individual A-scans are depicted as logarithmized gray scale or phantom color data. For example, a measuring time of one second will be needed for a B-scan consisting of 100 individual A-scans.
The measuring resolution of the OCT procedure is determined by the coherency length of the light source used and is typically about 15 μm. Due to its special suitability for examining optically transparent media the procedure is widespread in the field of opthalmology.
Two different kinds of OCT procedures have prevailed among those used in the field of opthalmology. With the first kind, the reference arm is modified in length to determine the measured data and continually measure the intensity of the interference without consideration given to the spectrum. This procedure is called “Time Domain” procedure. With the other procedure, called “Frequency Domain” procedure, however, the spectrum is considered in determining the measurements and the interference of the individual spectral components are recorded. Therefore, we refer to a signal within the time domain, on one hand, and to a signal within the frequency domain on the other.
The advantage of the frequency domain lies in the simple and quick simultaneous measuring where complete information about the depth can be determined without requiring movable parts. This increases both the stability and the speed.
The big technological advantage of the OCT is the decoupling of the depth resolution from the transversal resolution. In contrast to microscopy, this allows the recording of the three-dimensional structure of the item to be examined. The purely reflective and, therefore, contact-free measuring makes it possible to generate microscopic images of live tissue (in vivo).
Due to the high selectivity of the method very weak signals (less than a nanowatt) can be detected and identified to a certain depth. Therefore, the procedure is suitable for examining optically sensitive tissue. The use of the OCT procedures is limited by the depth penetration of the electromagnetic radiation into the subject to be examined, which is dependent upon the wavelength, as well as by the resolution, which depends upon the bandwidth.
With the currently customary biometric measuring devices, the measured data are processed in the device and suggestions are made as to the exchange lenses to be used. These depend upon the formulae used in the calculation and the type of available lenses (depending on the manufacturer). It is possible, or necessary, to let the post-operative results enter into the calculation formulae via the optimization of constants in order to allow for individual influences during the surgery as well as the measuring technique actually used. All measured values, data, and formulae are administered, analyzed, and saved in data banks and software programs. In part, these solutions are integrated in networks and various additional applications can be linked to them.
With the optical measuring devices based upon short coherence-light procedures, the interferometric principle based upon the dual-beam is used. This procedure is contact-free and works with the greatest accuracy currently possible. Solutions based upon this measuring principle have been described as examples in DE 198 12 297 C2, DE 103 60 570 A1 and WO 2004/071286 A1.
The disadvantages pointed out with the ultrasound devices can be avoided with the optical procedure. Special mention should be made of the high degree of accuracy (interferometer) and patient comfort. However, the disadvantage here is the fact that 10 to 20 percent of patients cannot be measured because, for example, the scattering of dense cataracts attenuates the measuring signal too much and the laser output cannot be increased at will due to the limits to be respected around the eye. In these cases it is also possible that the patient is no longer able to see the focal point and measuring becomes difficult.
Certain pathological changes can cause individual problems with determining the measuring data with both procedures. As a result of these negative influences upon obtaining the measurements there is an increased risk of making the wrong decision when selecting a suitable exchange lens.